1. Technical Field of the Invention
The present invention relates generally to load monitoring and more particularly to a method of determining loads and fiber orientations in anisotropic materials using energy flux deviation.
2. Discussion of the Related Art
In isotropic materials, the direction of the energy flux (energy per unit time per unit area) of an ultrasonic plane wave is always along the same direction as the normal to the wave front. In anisotropic materials, however, this is true only along symmetry directions. Along other directions, the energy flux of the wave deviates from the intended direction of propagation. This phenomenon is known as energy flux deviation and is illustrated in FIG. 1. An ultrasonic transducer U is coupled to an anisotropic crystalline material AM and directs an ultrasonic wave through the material as indicated by directional arrow N which is normal to the wave front. The anisotropic nature of the material causes the energy flux to deviate from the normal arrow N by an energy flux deviation angle A, resulting in an energy flux vector V associated with the deviated wave flow W. The direction of the energy flux is dependent on the elastic coefficients of the material. This effect has been demonstrated in many anisotropic crystalline materials. In transparent quartz crystals, Schlieren photographs have been obtained which allow visualization of the ultrasonic waves and the energy flux deviation.
The energy flux deviation in graphite/epoxy (gr/ep) composite materials can be quite large because of their high anisotropy. The flux deviation angle has been calculated for unidirectional gr/ep composites as a function of both fiber orientation and fiber volume content. Experimental measurements have also been made in unidirectional composites. It has been further demonstrated that changes in composite materials which alter the elastic properties such as moisture absorption by the matrix or fiber degradation can be detected nondestructively by measurements of the energy flux shift.
Graphite fiber-reinforced composites such as graphite/epoxy, graphite/magnesium and graphite/aluminum exhibit very high stiffness-to-weight and strength-to-weight ratios, making them excellent materials for lightweight aerospace structures. Since these structures are intended primarily to carry load, the in-situ and non-destructive determination of load and load induced quantities such as stress is very desirable. The energy flux of stress waves, i.e., waves affected by a load, propagating through anisotropic crystals has been shown to deviate from the direction of the normal to the plane wave. However, there has been no known indication that this deviation can be correlated with the amount of applied load. This lack of knowledge is not surprising since crystals are normally not used in load bearing applications. In addition, there is no known work concerning any energy flux deviation in highly anisotropic, non-crystalline materials such as graphite fiber-reinforced composites and consequently no known work concerning any effect of applied load on such deviations.